Anyone acquiring an image needs to have a permanent record which faithfully reproduces the original subject or scene, or at least those aspects of the subject or scene which are considered most important. The quality of the reproduction is judged by visually comparing the hardcopy with the original scene. In making this judgment an observer in part compares the brightness of various points in the hardcopy with the brightness of corresponding points in the original scene and thereby forms a subjective opinion about the quality of the reproduction. Exact subjective tone reproduction requires that the brightness of each point in the hardcopy equals that of the brightness of a corresponding point in the original scene. However, as those skilled in the arts know, exact subjective tone reproduction is extraordinarily difficult and inconvenient to achieve because hardcopy media from printers, and media from other devices (such as electronic cameras, scanners, monitors, etc.) are generally viewed at illumination levels which are significantly less than those under which an original was created--original photographs typically being about 1/100 that of the original scene. Also, most hardcopy or electronic media have a limited ability to capture the range of tones which typically exist in nature. Nevertheless, satisfactory subjective tone reproduction can, in part, be obtained if the perceptible brightness of the image on the medium appears similar to the brightness of the original scene.
To assure that the foregoing conditions are satisfied depends, in part, on properly matching the luminance range of the scene to the tone scale of the medium, taking into account the particular scene characteristics to be emphasized, prevailing scene lighting conditions, and the medium reproduction characteristics. The issue of matching the luminance range of the scene to the tone scale of the medium was addressed in the Scene Analysis method of U.S. patent application Ser. No. 08/414,750 filed Mar. 31, 1995 by James R. Boyack and Andrew K. Juenger. Specifically, an image is partitioned into blocks and certain blocks are combined into sectors. An average luminance block value is determined for each block and a difference is determined between the maximum and minimum average luminance block values for each sector. If the difference exceeds a predetermined threshold value, then the sector is labeled as an active sector and an average luminance sector value is obtained from maximum and minimum average luminance block values. All active sectors of the image are plotted versus the average luminance sector values in a histogram, then the histogram is shifted via some predetermined criterion so that the average luminance sector values of interest will fall within a destination window corresponding to the tonal reproduction capability of a destination application. The result of the Scene Analysis is a global balancing of the image brightness by large scale luminance adjustment.
Certain areas of the image typically still require luminance adjustments following Scene Analysis. These adjustments highlight and enhance specific regions of interest in the image in order to prevent loss of details and the subsequent reduced image quality. For instance, scenes with a large shaded region adjacent to a bright region are especially difficult to develop or display without loss of some image quality in either or both of the dark and bright regions. This type of luminance adjustment was addressed in U.S. Pat. No. 5,235,434 issued Aug. 10, 1993 to Munib Wober.
The method of patent '434 includes a first pass through an image compensation stage, the image being partitioned into groups of pixels called superpixels. Each superpixel is individually processed to determine whether it is part of a larger grouping (i.e. region) of superpixels which all fall beyond predefined darkness or brightness threshold values. Each superpixel is given a SIZE value and an AVERAGE luminance value for all pixels resident in the given superpixel.
In a second pass through the image compensation stage, the amount of luminance adjustment for each region of superpixels is determined in relation to both the corresponding SIZE and AVERAGE values. For small and intermediate regions, a smoothing function is applied to prevent the appearance of artifacts along the borders of the superpixel.
Further luminance adjustment is sometimes necessary at the pixel level. Thus, the present invention is primarily directed at an improved system and method for performing pixel level luminance adjustments of a digital image which is independent of large area luminance averaging. This and other objects will become apparent in view of the following description, drawings and claims.